1. TECHNICAL FIELD
The present invention relates to a process for recovering liquid hydrocarbons from a production wellbore in fluid communication with a reservoir having a gas cap overlying a liquid hydrocarbon producing zone, and more particularly to such a process wherein a foamed gel is injected into the reservoir to reduce gas coning into the production wellbore.
2. DESCRIPTION OF RELATED ART
A specific problem frequently encountered during the recovery of liquid hydrocarbons from a producing zone of a subterranean reservoir having an overlying gas cap is a phenomenon termed "gas coning". This phenomenon occurs when there is fluid communication between the producing zone and the gas cap across vertical flowpaths. Under sufficient drawdown pressure, the high mobility gas cap gas is drawn down from the gas cap through the vertical flowpaths into the producing zone where it commingles with the lower mobility liquid hydrocarbons residing therein.
Once in the producing zone, the gas cap gas tends to inhibit the flow of liquid hydrocarbons into the wellbore by preferentially flowing through the producing zone and entering the wellbore to the exclusion of the liquid hydrocarbons. Accordingly, gas coning is characterized by a significant increase in the gas/oil ratio of the produced fluids and an attendant significant decrease in the liquid hydrocarbon recovery rate from the production wellbore.
Gas coning treatments generally rely on placement of a gas blocking agent between the gas cap gas and the liquid hydrocarbon production wellbore to substantially block the vertical flowpaths for gas flow into the production wellbore. Such flowpaths are often either vertical fractures or gas-permeable reservoir matrix. U.S. Pat. No. 5,105,884 to Sydansk discloses a gas coning treatment employing a foamed gel as a gas blocking agent in vertical fractures extending between the gas cap and the liquid hydrocarbon producing zone. The foamed gel is made up of a crosslinked polymer gel, surfactant, and foaming gas. As a result of its high degree of structure, strength, and stability, the foamed gel effectively eliminates the gas permeability of the fractures. Foamed gels, however, are generally believed by the prior art to have no practical utility as gas blocking agents in vertical flowpaths of reservoir matrix. The placement of foamed gels in reservoir matrix is considered too difficult because of the relatively low permeability of the matrix and the relatively high degree of structure exhibited by foamed gels upon formulation.
As an alternative to foamed gels, the prior art teaches the use of conventional oilfield foams, which are gel-free foams consisting of only a surfactant solution and a foaming gas, for treating gas coning in matrix environments because such foams have a relatively low degree of structure enabling comparative ease of placement in the reservoir matrix. Unfortunately, the performance of conventional foams in the treatment of gas coning has been found inadequate. Whereas, conventional oilfield foams generally perform satisfactorily as dynamic mobility control fluids in hydrocarbon displacement floods, different performance demands are required of foams in the role of static gas blocking agents. When applied to gas blocking applications, conventional oilfield foams often do not have sufficient structural strength or stability to achieve long-term performance demands when continuously subjected to the gas flow pressure gradient existing during gas coning. Furthermore, conventional oilfield foams often do not have sufficient stability to maintain their structural integrity when subjected to downhole formation conditions typically encountered, particularly in the presence of liquid hydrocarbons.
As is apparent from the above, an effective gas coning treatment process is needed for blocking matrix flowpaths between a gas cap overlying a liquid hydrocarbon producing zone and a production wellbore in fluid communication therewith. Accordingly, it is an object of the present invention to provide a gas blocking agent that can be readily placed in matrix flowpaths, yet has sufficient structure, strength, and stability to satisfy long-term performance demands of the gas coning treatment process.